小猫 - 模棱两可的进口

Question

我几乎完全从这里找到的"派生秀"示例中获取了以下代码： https://github.com/milessabin/kittens

import cats._, cats.derived._, cats.implicits._
object Test extends App {
  case class Address(street: String, city: String, state: String)
  case class ContactInfo(phoneNumber: String, address: Address)
  case class People(name: String, age: Double,  contactInfo: ContactInfo)
  val mike = People("Mike", 1.23,  ContactInfo("202-295-3928", Address("1 Main ST", "Chicago", "IL")))
  implicit val addressShow: Show[Address] = new Show[Address] {
    def show(a: Address) = s"${a.street}, ${a.city}, ${a.state}"
  }
//  I would like to use a custom version of Show for Doubles...
//  implicit val doubleShow: Show[Double] = new Show[Double] {
//    def show(d: Double) = s"Custom Double Show: $d"
//  }
  implicit val peopleShow: Show[People] = {
    import auto.show._
    semi.show
  }
  println(mike.show)
}

我希望能够使用Show[Double]的自定义版本(我不是真的，但这是一个很好的最小示例，说明了我实际遇到的问题(

如果我取消注释我的doubleShow，则会出现以下错误：

Error:(25, 10) ambiguous implicit values:
 both value emptyProductDerivedShow in trait MkShowDerivation of type => cats.derived.MkShow[shapeless.HNil]
 and method emptyCoproductDerivedShow in trait MkShowDerivation of type => cats.derived.MkShow[shapeless.CNil]
 match expected type cats.derived.MkShow[A]
    semi.show

我怎样才能让它工作？我本来希望我的本地定义的隐式会胜过任何从猫进口的

Answer 1

尝试隐式对象而不是隐式 val

implicit object doubleShow extends Show[Double] {
  def show(d: Double) = s"Custom Double Show: $d"
}

或使用隐式 val 并更改导入(不要import cats.instances.double._(

import cats._, cats.derived._
import cats.syntax.show._
import cats.instances.string._ // People uses String and Double
implicit val doubleShow: Show[Double] = new Show[Double] {
  def show(d: Double) = s"Custom Double Show: $d"
}